Astronomers have found three groups of stars in the same open star cluster that seem to have different ages, challenging some of the fundamental principles used to estimate cluster ages.

Using NASA's Hubble Space Telescope to study the dimmest stars in open cluster NGC 6791, astronomers uncovered three differently aged populations, two of which are the burned-out shells of stars called white dwarfs. One group of these stellar remnants appears to be six billion years old, while the other appears to be four billion years old. The ages are out of sync with those of the cluster's normal stars, which are eight billion years old.

"The age discrepancy is a problem because stars in an open cluster should be the same age," says Luigi Bedin of the Space Telescope Science Institute in Baltimore. "They form at the same time within a large cloud of interstellar dust and gas. So we were really puzzled about what was going on."

In studying the dimmest burned-out stars in cluster NG 6791, NASA's Hubble Space Telescope has uncovered a paradox: three different populations of stars exist in an object where all the stars should have formed at the same time. Left: ground-based telescopic view of NGC 6791, located 13,300 light-years away in the constellation Lyra. The green inset box shows the view with Hubble's Advanced Camera for Surveys. Top right: The full Hubble Advanced Camera for Surveys field is full of stars estimated to be 8 billion years old. Two background galaxies can be seen at upper left. Bottom right: A blow up of view of a small region of the Advanced Camera for Surveys field reveals very faint white dwarfs. The blue circles identify hotter dwarfs that are 4 billion years old. The red circles identify cooler dwarfs that are 6 billion years old. Image: NASA, ESA, Digitized Sky Survey, and L. Bedin (STScI).

"This finding means that there is something about white dwarf evolution that we don't understand,” adds Ivan King of the University of Washington.

But after extensive analysis, the team of astronomers realised how the two groups of white dwarfs were fooling them. They recognised that if the stars were paired off in binary-star systems then that would make them appear as a single bright object from such a large distance, and this ‘extra’ brightness would make them look younger. Although binary systems also make up a significant proportion of the normal stellar population in NGC 6791, and are observed in many other clusters, this is the first time they have been found in a white dwarf population.

"Our demonstration that binaries are the cause of the anomaly is an elegant resolution of a seemingly inexplicable enigma," says Giampaolo Piotto of the University of Padova in Italy.

Now the team have just two ages to reconcile: an eight billion year age of the normal stellar population and a six billion year age for the white dwarfs. The connection might be in the form of a process that slows down white dwarf evolution.

“In present models that describe the evolution of white dwarfs, which is mainly a cooling operation, some physical process has not been properly accounted for that would provide an additional source of energy that the white dwarf would use to sustain its temperature longer,” Bedin tells Astronomy Now. “Since the hotter and brighter a white dwarf is, the younger it looks, meaning that these white dwarfs look younger than current models would predict. It is as if the white dwarfs were cheating on their age.”

The three populations of stars identified in NGC 6791 appear to be at different stages of evolution. Their positions have been indicated on this adaptation of the Hertzsprung-Russell diagram, which shows the relation between brightness and colour of the stellar populations. Two populations are white dwarfs while the remainder appear to be 'normal' main sequence stars. Image: NASA, ESA, and A. Feild (STScI).

Hubble's Advanced Camera for Surveys analysed the cooling rate of the entire population of white dwarfs in NGC 6791, which are the remains of stars between 1.1 and 7 times the mass of the Sun. The hot cores of white dwarfs radiate heat for billions of years and were thought to cool down at a predictable rate. As such, astronomers have used these seemingly perfect clocks as a reliable measure of the ages of star clusters. Now that idea has been turned on its head.

“It is a new territory,” says Bedin, “So far only two entire white dwarf cooling sequences of old clusters have been observed. We badly need more observations to understand more.”

Since the majority of stars in our Galaxy will end their life as a white dwarf star, understanding the evolution and cooling histories of more stellar remnants will give astronomers a clearer picture of the ages of the star clusters in which these ticking clocks reside.

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